Conventionally, beamforming is a technique for transmitting and receiving data using multiple antennas. In the beamforming technique, data is transmitted through multiple antennas while phase shifting control is executed to achieve directionality using the multiple antennas.
Multiple input and multiple output (MIMO) is a technique of transmitting data that differs according to system (see, e.g., Japanese National Publication of International Patent Application No. 2005-522086).
FIG. 23 is a diagram of a wireless communication apparatus that executes pre-coding MIMO. FIG. 23 depicts a partial configuration of a wireless communication apparatus of a base station in a mobile communication system. Data of multiple systems (4 systems in FIG. 23) are input into a pre-coder 901 and the phases are adjusted by given amounts to form a multi-beam. A transmitting unit 902 includes transmitting circuits 902a to 902d for the systems and transmits the data through antennas 903.
However, in each of the transmitting circuits 902a to 902d, variations in phase occur with respect to each analog element, arising in a problem in that although the beams are appropriately set at the pre-coder 901, the setting is lost at the transmitting unit and directionality cannot be achieved.
FIG. 24 is an explanatory diagram of a decrease in gain during multi-beam transmission. As depicted in the left portion of FIG. 24, when variations in phase have not occurred, directionality is effected by a multi-beam and gain is improved (by 6 dB) for transmission. On the contrary, when variations in phase occur in the transmitting circuits 902a to 902d of the transmitting unit 902, directionality becomes inadequate, e.g., expansion occurs, and the gain of each beam decreases (by 4 dB).
A condition for maximum gain (6 dB) to be obtained is the absence of variations in phase, i.e., all the antennas are in the same phase. The direction of each multi-beam is further uniquely determined. To satisfy this condition, for example, when four antennas are used, since the phases are adjusted to be same as that of a reference antenna, the phases of three of the antennas are adjusted.
As a condition for the gain to be high (6 dB) without determining the direction of each multi-beam is for relations of the phases among the antennas to be disposed along a straight line. To satisfy this condition when four antennas are used, the phases of two of the antennas are adjusted. When any of the conditions is not satisfied, a decrease in gain of each beam depicted in FIG. 24 occurs. The degree of the decrease in gain is determined by the degree that the phases of the antennas deviate from the straight line.
Conventionally, to solve the problem of variations in phase, using an external calibrating apparatus (or an internal unit having such a function) and by a comparison of the output from the transmitting circuits 902a to 902d of the transmitting unit 902 with a reference signal, the phases are adjusted such that no variations in phase occur.